Issues with synthetic rigging?

[echidna]

Hi John,

Thanks for your response. I would like to return to discussion of thermal expansion issues, which was one of the original topics of this thread. I will start a new thread to address the creep question.

As a starting point, there is the question of the coefficient of thermal expansion for Dynex Dux.

Do you believe the value I quoted above of -12E-6 per deg C (n.b., that's a negative value), which I found quoted in various places for Dyneema fiber, to be correct for Dynex Dux line? If not, what is the appropriate value?

[brian eiland]

Lets see, I spent about 45 minutes yesterday composing a posting with links for this discussion, only to loose the whole thing when I selected 'preview'. So before I go thru that again, I want to run a test.

[Joe Henderson]

Hi All,

Oh, good. This old chestnut.....again.

What all you data seekers need to realise is that Dyneema braid should be thought of as a structure, not a material and that all the data about UHMPE should be tempered with an understanding of how it behaves when braided/twisted/knotted.

The foremost authority on fibre and it's use in tension structures - Marlow... refuses to make any hard and fast claims about creep/stretch/thermal elasticity, solely because no one knows yet.

Use the fibre, embrace it's virtues and stop getting bogged down in minutae.

Talk to a RIGGER that is not out to make a quick buck from do it yourself rigging, and make your own mind up.

There are many many boats and rigs out there in Dyneema.

It is a legitimate material. It is tough. You wont break it. Your boat will benefit.

John Franta is to be trusted.

Regards,

Joe Henderson.

[Bob Oram]

Well said, Joe.

[Kim Klaka]

Hi,
I am a relative newcomer to this group; the answer to my question should be out there somewhere, but I haven't been able to find it. I have had some brief discussion with Brion Toss, but I still don't have a very convincing answer. The problem: why did my 2 year old spectra backstay break when I was still dockside? Some background...
My unsheathed Dyneema 6mm backstay (I believe it is SK75) from a reputable supplier broke after less than 2 years of light use. The failure occurred at the start of the upper eye splice when I applied the backstay purchase by hand when tied up in the pen. So far I have identified the following possible causes of failure, none of which seem entirely convincing to me.
1. Overloaded
The stay was definitely not overloaded – probably less than 30% of breaking load at the time (see end-notes 2 and 3 for details).
2. Chafe
There is nothing near the backstay up at the top splice so chafe is almost impossible to occur. A rigger specialising in synthetic rigging said it might be the cockatoos (parrots) pecking it. This happens to the dinghies in the dinghy park but I have never seen the cockatoos on the jetties here and there was no evidence of beak damage on the broken stay.
3. Badly made splice
The splice was longer than standard recommendation, though the amount of taper was not very much from what I can tell (Brion notes this is an important issue). It was made by a rigger who does a lot of plastic rigging work on both racing and cruising yachts, and has a good reputation.
4. U-V degradation
This is my main concern. There is a lot of variation in the data about how Dyneema etc. degrades with U-V exposure. Some reliable sources suggest a gradual decrease in strength by 20- 30% over 2 years, then stable thereafter (Brion mentioned similar figures). The stabilisation is probably due to the oxidised material forming a protective barrier over time, a bit like the oxide on aluminium alloy. I guess the actual figures depend on the amount and type of U-V stabiliser added to the rope. The u-v out here in Western Australia is truly fierce (the annual average noon u-v index is 8, and over the summer it averages 12 with many days over 13. I understand this is worse than Miami and much worse than the Med).
5. Fatigue
I have not investigated this possibility, but I noticed that one of the disadvantages of switching from a stainless wire to a braided rope backstay is the humming noise off the backstay in the wind. Given that the tension and the windspeed are the same for both materials, the two differences are the diameter and the surface roughness of the rope/wire. This may affect the Karman vortex street generated, which is a main cause of vibration, which in turn is a source of fatigue cycling. Fatigue properties of Dyneema?.....

My solution has been to revert to a stainless steel backstay seeing as I never has a strength issue with it. That is a pity given all the claimed advantages of dyneema, so I’d like to get to the bottom of this puzzle. The most likely reason I can come up with so far is a combination of a less than ideal splice, severe u-v degradation and possibly a helping hand from fatigue cycling. If that’s the case, we have a time bomb of rigging failures for uncovered spectra ticking quietly away here in sunny Western Australia.

Note 1: Materials selection criteria
An adjustable backstay is a different application from other standing rigging because stretch and creep are largely irrelevant, it is just a weight v windage trade-off (plus longevity and price).
Note 2 – Load calculation:
I applied the 24:1 backstay purchase by hand on my Van de Stadt 34 (when tied up in the pen). I estimate the load at failure (50kg*24=1200kg, less block friction) was at most 40%, of breaking load more likely 25% (see note 3 below).
Note 3- breaking load:
It is difficult to determine the true breaking load of the rope used - the supplier quoted 2,900 kg; Amsteel-blue shows 3,500kg; Melbourne Rope and Splicing Dutch SK75 says 4,200kg. I suspect the huge range may in part be due to some of the figures being safe working load whilst others are breaking load, or one of the many variants in between. Suppliers can sometimes be ambiguous in their definitions, and retailers are much worse. The bottom line is that the 7/32'' 1x19 316 stainless backstay at 2,450kg " breaking load" has worked fine for 30 years of use on this boat (rigging replaced every 8-10 years), so I should be able to account for safety factors etc. by sizing the plastic rope using the stainless as my benchmark

[Brion Toss]

Hello,
Thanks for all the details. For the information of most of the people reading this, what follows is a follow-up to an earlier contact by email, and a delightful Q and A, via Skype, with the Freemantle Sailing Club.
So, to take things in order, "probably less than 30% is distinctly overloaded; 20% is a good design load for most rope (or wire rope), and 10% is good to shoot for with Spectra, in order to minimize creep.More on this later.
Glad to hear that the cockatoos hadn't been attacking your rope. We don't get a lot of that here in Port Townsend, either, though we do have problem with bird poop in blackberry season.
The splice does not appear to have been tapered at all (I have a picture under separate cover). According to Starzinger's tests, this could weaken the rope by at least 15%.Note also that, as creep progresses, the rope is somewhat weakened (https://www.dsm.com/content/dam/dsm/...plications.pdf). So now we have a stress riser, plus the effects of creep.
Next, a biggie for Spectra: UV. I agree that this is a time bomb in your part of the world; after two years with no treatment, uncovered Spectra could easily lose 25% of more of rope strength.
Finally, note that it is not you who applies load to that backstay, it is the wind. I recommend having someone monitor a tuning gauge next time you are under way, so see what the loads get to.
In sum, we have a not-great splice, plus undersized rope, plus time in the sun, plus uncertainty on loading. It does not seem surprising that the rope failed.
Try this with your wire backstay: use cable clamps instead of swages for your terminations. Use an undersized thimble, for a too-tight radius. Apply some tape along the standing part, to trap water and salt inside. In other words, do the things that are known to weaken wire rope. Of course that is a bad idea, but that kind of describes what happened to your Spectra backstay. I recommend HSR or similar, not Amsteel blue, for standing rigging, and I recommend sizing it appropriately. For your part of the world, a covered version seems important; you'll still save considerable weight.
Fair leads,
Brion Toss

[Kim Klaka]

Great forum! Thanks indeed for your further thoughts Brion, and for putting the issue in perspective for the general reader. I still think there is something unexplained going on, and would appreciate further ideas from yourself and others.
So as I understand it for the general reader we have to note that:
1.
Whilst variations in splicing technique will lead to fluctuations in breaking load, so do fluctuations in wire swage tolerances. And its probably a lot easier to tell if a splice has been done badly than it is to tell if a swage has been done badly. Advantage plastic rigging.
2.
Both plastic rigging and stainless wire degrade in the environment - plastic due to u-v and wire due to salt etc. (however, the degradation rate in wire increases as it gets older whereas the degradation rate in plastic is greatest in the first few years then levels off). U-V degradation of Dyneema implies a covering is advisable in extreme sunlight. No advantage to either plastic or steel, just different.

Then to the specific case of the broken spectra backstay:
The load in this case was not applied by the wind, it was applied by me pulling the backstay tackle on in the pen before we even put the sails up. This is just a standard cruiser-racer with a standard block and tackle. I have sailed on lots of boats that apply a very much higher pre-tension in their (wire) backstay. Without revisiting the sums, the bottom line is that the spectra broke when the backstay tackle was applied whereas the wire never broke under very much higher loads exerted by the exact same backstay tackle plus the much bigger loads of wind and waves. This despite the plastic rope having a higher quoted breaking load than the wire. Even when you consider all the factors (u-v –salt corrosion, splice quality, swage quality, creep-induced weakness etc.) it just doesn’t add up that a stronger plastic rope breaks at a hugely lower load than a weaker stainless wire.

There’s something missing in the analysis which needs a bit more investigating. Suggestions, anyone?
Kim